1. Field of the Invention
The present invention relates to fluid containers having elastomeric seals between enclosure components and, more particularly, relates to an improved portable piston style sample cylinder having reduced seal permeability to pressurized fluid within the sample cylinder.
2. Description of the Background
Portable sample cylinders for collecting and transporting fluid samples have long been used in the liquid petroleum gases (LPG) industry. Such sample cylinders commonly include a piston movable within the cylinder, which allows the fluids to be collected and transported at the pressure of the fluids when sampled, so that the physical state of the fluid is not altered. Since the sampled fluid is the basis for subsequent tests to determine the constituents or caloric content of the fluid, care must be taken that no constituents of the sample leak from the sample chamber. Moreover, conventional seal lubricants tend to absorb certain chemical groups within the LPG or other sampled fluid, and thus are generally avoided. A representative portable piston sampler and background relating to such samplers is disclosed in U.S. Pat. 4,862,754, hereby incorporated by reference.
Problems with respect to the integrity of the fluid are heightened when the sample cylinder as shown in the '754 patent is used to transmit "speciality gases." Speciality gases are extremely expensive since the constituents of the fluid are carefully controlled. Such gases are commonly used as a standard to verify the accuracy or to calibrate precision test equipment, such as chromatographic equipment. Moreover, such speciality gases frequently employ constituents having very small molecules, and these molecules tend to escape from the container at substantially different rates than larger molecules constituents. If the vapor pressure of the "high end" gas is 800 PSI at room temperature, the speciality gas is typically maintained at a slightly higher pressure, e.g. 850 PSI, to prevent vaporization. A pressurized gas may be housed within the sample cylinder separate from the speciality gas, and the piston within the cylinder separates these fluids while maintaining the speciality gas within its desired pressure range.
A further problem with sample cylinders used to transmit LPG and other fluids, and particularly for those vessels used to transmit speciality gases, is the permeability of the seals used to seal container components. Since some of the sample constituents chemically react with most if not all elastomeric seal materials differently than other sample constituents, and since some constituents are more easily absorbed into the elastomeric seal material than other constituents, the permeability of the seal can have a significant effect on the integrity of the fluid in the cylinder. The integrity of a small quantity of speciality gas costing thousands of dollars can be destroyed by an elastomeric seal which (a) allows some portion of the pressurized fluid to escape from the container, (b) chemically reacts with some constituent of the fluid, or (c) alters the composition of the sample by allowing one or more constituents to become absorbed into the seal material.
The disadvantages of the prior art are overcome by the present invention, and an improved portable sample cylinder with reduced seal permeability is hereinafter disclosed for maintaining the integrity of the sampled fluid. Also disclosed is an improved method to manufacture a sample cylinder to achieve the benefit of reduced seal permeability.